The question on most people’s minds now is, what will the rest of the crop season be like? Uppermost in the thought process is wondering when the first frost or killing freeze will occur.
Beth Hall, Indiana state climatologist, previously managed the Midwestern Regional Climate Center, based in Champaign, Ill. The MRCC collects data for multiple states, including Indiana.
When looking at the probability of whether a frost or freeze will come early, the first order of business is to understand terms. The MRCC typically charts dates and probabilities for a light frost, or when the temperature reaches 36 degrees F; a freeze, or 32 degrees; and a killing freeze, which for most plants is 28 degrees.
For corn, Bob Nielsen, Purdue University Extension corn specialist, says the changes that occur within the corn plant at 28 degrees normally cause it to die. At temperatures above 28 degrees, the plant may survive. However, there may be little time left for the crop to dry in the field.
Here are examples using historical information from 1990 through 2019 at two locations in Indiana: Columbus and Columbia City. Numbers in the table below were extrapolated from graphs in the MRCC database.
For example, note that at Columbia City, the probability of a 36-degree frost by Sept. 22 is 10%. There is a 50% chance frost will occur by Oct 4. The probability of a 32-degree freeze by Sept. 23 is also 10%. There’s a 50% chance by Oct. 17. However, the probability of a 28-degree, corn-killing freeze by Oct. 5 is only 10%. There’s a 50% chance that a 28-degree reading will occur by Oct. 21, and a 90% chance by Nov. 4
Around Columbus, it’s a different story. There’s a 10% chance for a 36-degree frost by Sept. 25, a 50% chance by Oct. 12 and a 90% chance by Oct. 26. There is only a 10% chance you would see 32 degrees by Oct. 10, a 50% chance by Oct. 23 and a 90% chance by Nov. 2. There’s only a 10% chance you will see a killing freeze by Oct. 25, a 50% chance it won’t happen before Nov. 4, and a 90% chance it will happen by Nov. 15.
Hall says you can find the median date of the first 32-degree or 28-degree freeze for Indiana online. More specific freeze date climatology can be found on MRCC’s cli-MATE online data portal. It was used for the previous examples. It’s free to the public, and registering an account is quick and easy, Hall says. Once you register and log in, click the “Select Daily Station” button found in the top banner and use an interactive map to find a station of interest.
After selecting the station, using the navigational panel on the left-hand side of the main page, select “Daily-Observed Data,” then “Daily,” then “Threshold Search,” Hall says. You can select the years and subperiods of interest along with the threshold criteria you want.
For the probabilities of a freeze occurring before, after or on particular dates, go to “Daily-Observed Data,” then “Seasonal,” then “Freeze Probabilities.” You will find charts like those used to prepare the table shown here.